Algebraic data types for C99
Java 8 annotation processor and framework for deriving algebraic data types constructors, pattern-matching, folds, optics and typeclasses.
Monad, Functional Programming features for Golang
Fast and memory-efficient immutable collections and helper data structures
Algebraic data types for Python (experimental, not actively maintained)
An implementation of sum types in Julia
nextgen MLStyle: Generic Algebraic Data Type + Pattern Match
Recursive Variant: A simple library for Recursive Variant Types
A type-safe and space-efficient sum type for C# (comparable to discriminated unions in C or C++)
Easy-to-use sum types in Julia
Expressive High-Performance Eager Functional Programming Library for C++ 11
ftor enables ML-like type-directed, functional programming with Javascript including reasonable debugging.
Tagged unions for Elixir. Just that.
A code generator enabling sum-types in Dart
Result, Either, Try, and Maybe monadic types for Java
Functional Enum type / Sum type for javascript with simple pattern matching
Track periodic goals — a sample iPhone app
A namedtuple-style library for defining immutable sum types in Python.